Liquid crystal display device with birefringence compensator and volume hologram

ABSTRACT

A display device comprising a liquid-crystal cell element ( 20 ) placed between two polarizers ( 23, 28 ) comprising at least one optical structure for compensating for the variations in birefringence of said liquid crystal according to the viewing angle. The optical compensation structure comprises at least one oblique-axis film ( 25, 26 ) of polymerized liquid-crystal type suitable for at least partly compensating for the undesirable effects of the natural birefringence of the liquid crystal, combined with a volume hologram ( 24, 27 ) of small retardation suitable for improving the compensatability of said film.

The present invention relates to liquid-crystal display devices withcompensation of the birefringence, allowing the viewing angle of thedisplay device to be considerably increased.

It applies especially in electrooptic display devices and morespecifically in liquid-crystal displays, used in transmission, inreflection or even in projection on a screen.

Liquid-crystal screens have experienced very substantial growth with thedevelopment of portable computers using TFT (Thin Film Transistor)technology and a TN (Twisted Nematic) liquid-crystal cell.

Most liquid-crystal displays or screens suffer from a major drawback,namely the limited viewing angle at which they can be observed: upondeviating from the normal to the surface of the display or screen, thecontrast between black and white decreases considerably and the imagepresented deteriorates. This is because, owing to the intrinsicbirefringence of the liquid crystal, the contrast level drops as soon asthe observer deviates from the normal to the screen and, for certainareas of observation, the gray levels are inverted.

This phenomenon, acceptable for some applications, must absolutely becompensated for when it is a question of producing computer screens orany display devices that can be consulted by several observers at thesame time.

The viewing angle properties of a screen or display of the LCD(liquid-crystal display) type are generally evaluated using a conoscopewhich gives the isocontrast curves as a function of the angle ofobservation, characterized by the following two angles:

-   -   θ=angle of the observer with respect to the normal of the        screen;    -   φ=angle of projection of the direction of observation in the        plane of the screen, with respect to the East-West (horizontal)        axis.

FIG. 1 depicts the conoscope of an uncompensated TN cell. This conoscopeshows that the range of viewing angles for which the contrast forexample greater than 50 is small.

The prior art discloses various methods and structures whose objectiveis to remedy the aforementioned problem.

1) Multidomains

A first approach consists in modifying the structure of the cell bycreating, in each elementary cell (pixel), several domains in which theanchoring of the liquid crystal is different. The averaging effect thusobtained reduces the problem substantially, but leads to an increase inthe complexity in the process for manufacturing the screen.

2) Novel Electrooptic Effects

A second approach consists in using other types of liquid cells in whichthe alignment, the nature of the liquid crystal or the addressingprinciple are different from a TN (twisted nematic). Some of them, suchas IPS (In Plane Switching), have resulted in commercial products havingproperties equivalent to that of a TN and possessing a large viewingangle. However, these cells are based on complex effects which are notalways under control in the manufacture of LCD screens.

3) Birefringent Films

A third approach does not modify the structure of the cell but correctsthe birefringence of the liquid crystal by adding one or more optimizedbirefringent films in order to compensate for the effect of the liquidcrystal. The compensation philosophy is the following: the problem ofthe viewing angle of liquid-crystal cells stems from the birefringentcharacter of the liquid crystal, which converts the polarization oflight wave differently according to its angle of incidence. Sinceextinction between cross polarizers is only possible if the outputpolarization is linear, black is obtained only for angles close to thenormal to the screen. The addition of films possessing an “inverse”birefringence makes it possible to reduce, or even eliminate, thisbirefringence.

Comments on Birefringent Films

A birefringent medium is characterized by its index ellipsoid, thesurface characteristic of the index of propagation of a lightwave ofgiven polarization and of given direction: the axes of the intrinsicframe of the ellipsoid constitute the intrinsic axes of the medium andthe length of these axes is equal to the index of propagation of thelight polarized along the corresponding axis.

If the ellipsoid is of circular cross section, the medium is “uniaxial”.For a uniaxial medium, the index along the axis of revolution or opticalaxis is called the extraordinary index n_(e) and the index along theother two axes is the ordinary index n_(o), as depicted in FIGS. 2 a and2 b. If the extraordinary index n_(e) is greater than the ordinary indexn_(o), the medium is called a positive uniaxial medium and the ellipsoidis elongate, in the form of a “cigar” (FIG. 2 a). The extraordinary axisis the slow axis. Conversely, if the extraordinary index n_(e) is lessthan the ordinary index n_(o), the medium is called a negative uniaxialmedium and the ellipsoid is flattened, in the form of a cushion or“dish” (FIG. 2 b). The extraordinary axis is the fast axis.

The difference between these two indices is very small, for example ofthe order of 1%, but it is sufficient to introduce very large changes inpolarization.

The medium is biaxial if the ellipsoid is not a body of revolution, thatis to say there are three orthogonal intrinsic axes with three differentindices.

The inclination of the optical axis for a uniaxial medium is indicatedby the angles (θ,φ) in which:

-   -   θ is the angle of the optical axis with respect to a        perpendicular to the plane of the screen;    -   φ is the projection of the optical axis in the plane of the        screen with respect to the east-west direction.

The retardation R_(o) of a uniaxial film is defined as follows:

-   -   R_(o)=(n_(e)−n_(o))d, where d corresponds to the thickness of        the film.

If R_(o)>0, the film is called a positive uniaxial film.

If R_(o)<0 the film is called a negative uniaxial film.

Liquid crystals are positive uniaxial media, the optical axiscorresponding to the director of the liquid-crystal molecule.

There are many combinations of uniaxially or biaxially basedbirefringent films allowing the viewing angle of a liquid-crystal cellin general, and a twisted nematic or TN cell in particular, to beimproved.

The birefringent films are generally positioned between the polarizersand the substrates of the cell, in various geometrical configurations.

FIG. 3 shows an example of an arrangement of birefringent films forcompensating a liquid-crystal cell in terms of viewing angle.

The liquid-crystal cell 1 is positioned between a first assembly formedby a polarizer 10 and by a compensator 11, which consists for example ofa birefringent film, and a second assembly formed by a compensator 12and by an analyzer 13. The reference 14 denotes the substrate of theliquid-crystal cell.

Various methods exist for obtaining birefringent films, some of whichare given below by way of illustration but not at all implying anylimitation.

Stretched Plastic Film.

By uniaxially or biaxially stretching a plastic film (PVA, short forpolyvinyl alcohol, polycarbonate or CTA, short for cellulosetriacetate), it is possible to obtain any “in plane” birefringence(negative uniaxial type, positive uniaxial type, biaxial type), that isto say with the axes of the index ellipsoid lying within the plane ofthe film or along the normal. However, at the present time, thistechnology does not allow inclined optical axes to be obtained. As it isoften necessary to provide birefringent films with optical axis inclinedwith respect to the plane of the film, the performance of thecompensators is limited thereby.

The retardations obtained by this technology are of the order of −100 nmand more. For some orientations of the optical axis of the film, it isdifficult with this method to obtain low birefringence values, and to doso reproducibly, for example for values falling within the [−20, −60 nm]range.

Oblique Films

A known effective technique for compensating for the positive-typebirefringence of a nematic liquid-crystal cell consists, for example, inusing a negative-type birefringent film. For example, in order tocompensate a TN cell effectively, it is necessary to provide anegative-type film whose optical axis is inclined with respect to theplane of the substrate.

Fuji Film

It is known to couple, on each side of a TN cell, a continuum consistingof an oblique negative uniaxial medium and an “in-plane” positiveuniaxial medium generally obtained by stretching a plastic film. Such acompensation film is disclosed, for example, in U.S. Pat. No. 5,583,679or in the publication [1] entitled “Application of a negativebirefringence film to various LCD modes” by N. Mori et al, ProceedingsSID 97, pp 11–88.

The inclined negative uniaxial medium continuum is obtained usingpolymerized discotic liquid-crystal molecules. This method ofcompensation has given rise to a film sold by Fuji and denoted in therest of the description by “Fuji film”.

The Fuji-type solution therefore comprises a continuum of a negativeuniaxial medium consisting of a splay of polymerized discoticliquid-crystal molecules and of a negative uniaxial medium of opticalaxis perpendicular to the substrate obtained via a plastic (CTA) film,which is also the substrate of the polarizers used in display, asexplained in the aforementioned publication [1].

In its current commercial version, the Fuji film consists of a stackcomprising a CTA (cellulose triacetate) and of a layer of polymerizeddiscotic molecules, this being depicted in FIG. 4.

The CTA substrate 20 of the Fuji film is a stretched plastic film of thenegative uniaxial type, the optical axis of which is perpendicular tothe plane of the layers. To use the Fuji film correctly, it has to becombined with a polarizer that itself has a CTA substrate ofwell-defined retardation, the CTA of the polarizer and the CTA of theFuji film being bonded together. The values of the CTA retardations arearound −40 nm for example.

The multilayer recommended by Fuji for obtaining good compensation mustinclude a layer of polymerized discotic molecules 21 and a negativeuniaxial film of optical axis perpendicular to the plane of −80 nmretardation.

The structure of the layer of polymerized discotic molecules isillustrated schematically in FIG. 4.

The Fuji film is a compensation film developed for compensating a TNliquid-crystal film having a thickness d of 4.7 μm. The liquid-crystalmolecules become increasingly inclined on going away from the polarizer.Their optical axes are initially inclined at an angle α of 4° to thenormal to the surface, attaining a final angle of inclination of 68°.Each film compensates a liquid-crystal half-cell. The compensation ofsuch a liquid-crystal cell therefore requires the use of a Fuji filmplaced on each side of the cell.

The compensation principle is based on the fact that since the discoticmolecules have the reverse birefringence of the nematic moleculesconstituting the liquid-crystal cell, each discotic molecule compensatesfor a nematic molecule of parallel optical axis.

FIG. 5 shows the isocontrast curves for a liquid-crystal cellcompensated by a film sold by Fuji. A Fuji-type film is placed on eitherside of the liquid-crystal cell.

Holography

Another method for obtaining a negative uniaxial medium is based on theuse of a holographic grating. When the fringe spacing is small enoughcompared with the illumination wavelength, the hologram operates in formbirefringence mode and is equivalent to a negative-type uniaxial mediumwhose optical axis is coincident with the normal to the fringe plane.Such a correction method is described, for example, in the patents FR 2754 609 and FR 2 778 000 or else in the document entitled “TN-LCDviewing angle compensation with holographic volume gratings” by C.Joubert et al., Photonic West '99 SPIE Proceedings, No. 3635, 137-142(1999).

4) Improvement of Films

The properties of the abovementioned films, in particular the Fuji filmusing polymerized discotic molecules, may also be improved by adding afilm of small retardation, of around 20 to 50 nm, with a givenorientation. Thus, it is possible to combine the aforementioned Fujifilm with a negative-type film of about −25 nm retardation with theoptical axis lying, for example, in the plane of the substrate. Adifferent orientation of the optical axis is also possible. An exampleof such a structure is given in the article entitled “Improvement of theoptical characteristics of a twisted-Nematic Display using negative inplane and splayed discotic films”, Proceedings of the SID '98, p. 694 byT. A. Sergan and J. R. Kelly. This article recommends the use of astretched film in the plane having an optical axis lying within theplane of the screen.

The concept of the invention consists especially in combining a volumehologram with a Fuji-type compensation film so as to improve thecompensatability of the film. In particular, the characteristics of thevolume hologram are chosen depending on the film whose compensatabilityis to be improved.

The invention considers an optical axis of any inclination for the film.

The invention also opposes, in addition to using a holographic film, tooptimize the CTA of the Fuji film, the combination of the twoimprovements (holographic film+CTA optimization) very substantiallyimproving the performance of the Fuji film.

The present invention relates to a display device comprising aliquid-crystal cell element placed between two polarizers, comprising atleast one optical structure for compensating for the variations inbirefringence of said liquid crystal according to the viewing angle. Itis characterized in that said optical compensation structure comprisesat least one oblique-axis film of polymerized liquid-crystal typesuitable for at least partly compensating for the undesirable effects ofthe natural birefringence of the liquid crystal, combined with a volumehologram of small retardation suitable for improving thecompensatability of said film.

The oblique-axis film may be of the nematic or polymerized discoticliquid-crystal type.

The hologram is, for example, a holographic film having an optical axisin the plane of the oblique-axis film or else a holographic film havingan optical axis tilted with respect to the plane of this film.

The value of the retardation of the said volume hologram, expressed inabsolute value, is for example less than −150 nm, preferably between −10and −100 nm.

The holographic assembly may consist of at least two holographic filmseach comprising a multilayer grating the layers of which have their ownorientation.

According to one embodiment, the oblique-axis film comprises, forexample, a stretched plastic film of the CTA type having a negativeuniaxial birefringence and in that the retardation value of this film istailored to the holographic film and to the oblique-axis film.

The liquid-crystal cell is, for example, of the twisted nematic type.

The device according to the invention is used, for example, tocompensate for the birefringence effects in display devices such asmicrocomputer screens.

The invention has in particular the advantage of improving thecompensation provided by the existing birefringent films used instructures for compensating for birefringence effects.

Further advantages and features of the invention will become apparent onreading the detailed description which follows and which is given withreference to the appended drawings in which:

FIG. 1 shows the conoscope of an uncompensated TN cell;

FIGS. 2 a and 2 b, the ellipsoids of a positive-type uniaxial medium anda negative-type uniaxial medium, respectively;

FIG. 3 shows schematically a structure of a compensated cell;

FIG. 4 shows schematically the structure of the Fuji-type film and

FIG. 5 a conoscope obtained for a liquid-crystal cell compensated usingsuch a film;

FIG. 6 depicts a first scheme for a cell compensated using a structureaccording to the invention;

FIG. 7 is a second scheme for a cell compensated according to theinvention;

FIG. 8 is an exploded view of the structure depicted in FIG. 6,comprising a Fuji film and a negative uniaxial medium in the holographicplane, and

FIG. 9 is the conoscope obtained by this structure;

FIG. 10 is an exploded view of a structure comprising a Fuji film and aholographic oblique negative uniaxial medium, and

FIG. 11 the conoscope obtained by this structure;

FIG. 12 is an exploded view of a structure according to the inventioncomprising a Fuji film having an improved CTA and a holographic obliquenegative uniaxial film, and

FIG. 13 the conoscope obtained by this structure.

It has been discovered that combining a volume hologram havingjudiciously determined parameters with a film of inclined optical axisor oblique axis, such as the Fuji film, considerably improve thecompensatability of the film. In addition, optimizing the CTA of theFuji film also improves the conoscope obtained with the Fuji/hologramcompensation structure.

FIG. 6 shows in a simplified manner an example of such a construction.The orientation of the device is made with respect to the East-West andNorth-South directions indicated at the bottom of the figure.

The liquid-crystal screen 20 comprises, for example, a twisted nematicliquid crystal. This liquid crystal is sandwiched between two glassplates 21 and 22 (not shown in this figure), the faces of the platesthat are in contact with the liquid crystal having been treated byrubbing so as to define the orientation of the molecules in contact withthese faces and their tilt with respect to the plane of the faces.

The liquid crystal is placed between a first polarizer 23, a volumehologram 24, a first film 25, for example of inclined optical axis, soldby Fuji and a second film 26, substantially identical, for example, tothe first film 25, a volume hologram 27 and a second polarizer 28. Thetwo polarizers are oriented, for example, at 90° to each other, possiblyto within a few degrees thereof.

The invention also applies to all the compensation films comprising atleast one birefringent medium whose optical axes are oblique (withrespect to the plane of the liquid-crystal screen) or inclined in theplane of the film, for example films of inclined optical axis of thepolymerized nematic liquid-crystal type that are currently notcommercialized.

Holographic Film or Negative Uniaxial Film

The volume hologram is a holographic film in which an index grating hasbeen recorded in the volume. In such a film, the optical axis iscoincident with the normal to the plane of the index layers.

The characteristics of this holographic film are defined so as inparticular to improve the compensatability of the film of inclinedoptical axis.

The holographic film is equivalent to a negative uniaxial medium andoperates in form birefringence mode. It possesses artificialbirefringence properties in the case of wavelengths much longer than thespacing of the layers forming the grating. The sinusoidally modulatedindex layers are spaced apart with a smaller spacing than the wavelengthwhich passes through them. These layers constitute nondiffractingholograms for the light used for the liquid crystal.

The fringes may be created by ultraviolet light interference in aphotosensitive material. The recording process used is, for example,described in the Applicant's patent FR 2 778 000.

Retardation of the Film

The equivalent retardation R_(h) of the holographic grating as afunction of the modulation of its refractive index Δn is obtained, forexample, from the following simple formula:

$R_{o} = {{- \frac{\Delta\; n^{2}}{n_{o}}} \times d}$with

-   -   d: thickness of the film    -   n_(o): mean index    -   Δn: modulation of the refractive index.

For example, a typical value for Δn for producing a holographic gratingof a photopolymer manufactured by DuPont is equal to 0.045. Taking atypical film thickness of 25 μm and a mean index n_(o)>1.5, gives aretardation value R_(h) of −40 nm. The retardation is calculated usingthe method described for example in the publication entitled “TN-LCDviewing angle compensation with holographic gratings” by C. Joubert etal., Photonic West '99, SPIE Proceedings No. 3635, 137–142 (1999).

Each film may have a retardation of less than −150 nm and preferablyfalling within the [−10 nm to −100 nm] range of values.

Angles

The orientation of the optical axis of the holographic film lies in theplane of the Fuji film or is tilted with respect to the plane of thisfilm. The angle of orientation of the index layers is defined accordingto the characteristics of the film.

The angle of inclination of the optical axis of a holographic film canmake an angle θ of between 0° and 90° with the normal to the plane ofthis film.

The projection of the optical axis of the holographic films on the planeof the film makes an angle φ of between 0° and 360° for example.

The angles θ and φ associated with each holographic film will beoptimized by simulation, in order to produce an effective compensator.

The parameters that it is desired to optimize are especially:

-   -   the retardation Rp of the “in plane” negative uniaxial film for        an angle θ of approximately 0° (optical axis perpendicular to        the plane of the screen);    -   three values (R_(F), θ_(F), φ_(F)) corresponding, respectively,        to the retardation of the oblique negative uniaxial film        produced by holography, to the angle θ_(F) of its optical axis        with respect to the normal to the screen, and to the angle φ_(F)        of the projection of the optical axis in the plane of the        screen.

If the compensator comprises several holographic films, each associatedtrio (R_(F), θ_(F), φ_(F)) must be optimized when taking the entireapplication into account.

The compensator is optimized by using, for example, a program with thename DINOS sold by Autronic-Melchers GmbH capable of modeling theoptical transmission of a multilayer comprising a liquid-crystal cell, aFuji film including a CTA, and a holographic film.

The optimization is accomplished, for example, by displaying thecontrast conoscope obtained for a given configuration and by looking forthat variation in the compensation films that will improve the viewingangle. The final solution—a better compensation structure—is obtained bysuccessive approximations and iterations.

The holographic assembly may include one or more films each having amultilayer grating, each layer having its own orientation.

It is also possible to modify the CTA of negative uniaxial axis in theplane of the Fuji film. The modification consists, for example, instretching the existing CTA for the commercial Fuji film so as to varythe value of the retardation, depending on the compensation to beapplied to the Fuji film. This variation is introduced into theoptimization steps described above.

FIG. 7 shows an alternative embodiment of FIG. 6, in which a holograph24, 27 is bonded to each of the faces of the liquid-crystal cell 20, andthe Fuji film 25, 26 is placed between a polarizer 23, 28 and a hologram24, 27.

FIG. 8 shows an exploded view of the diagram of FIG. 6, comprising anon-uncrossed Fuji film combined with a negative uniaxial film having aretardation value of −25 nm.

The liquid-crystal screen 30 comprises, for example, a twisted nematicliquid crystal. This liquid crystal is sandwiched between two glassplates 31, 32 whose faces in contact with the liquid crystal have beentreated by rubbing so as to define the orientation of the molecules incontact with these faces and their tilt with respect to the plane of thefaces. The direction of rubbing on the face 31 is at −45° to theWest-East direction. The direction of rubbing on the face 32 is at +45°to the same direction. Depending on the configuration, the polarizer 33associated with the face 31 is oriented at 90° to the direction ofrubbing on this face. The polarizer and the analyzer are thereforeoriented at 90° to each other, possibly to within a few degrees thereof.

The analyzer 34 associated with the face 32 is oriented at 90° to thedirection of rubbing on the face.

The compensation structure bonded to the face 31 comprises, for example,a first holographic film 35 (a negative uniaxial film) having aretardation value of around −25 nm, angles θ and φ having values of 90°and 135° respectively, a Fuji film 36 as described above, having anangle φ with a value of 315° and its CTA 37 having a retardation valueof −80 nm and an angle θ of 0.

The compensation structure bonded to the face 32 comprises, for example,a second holographic film 38 having a retardation value of around −25nm, angles θ and φ having values of 90° and 45° respectively, and a Fujifilm 39 as described above having an angle φ with a value of 225° andits CTA 40 having a retardation value of −80 nm and an angle θ of 0.

FIG. 9 shows the isocontrast curves of the compensated cell described inFIG. 8. The conoscope shows an improvement in the compensatabilitycompared with the conoscope of FIG. 4, representative of the commercialFuji film.

FIG. 10 is an exploded view of an example of a structure according tothe invention in which the compensatabilities of the commercial Fujifilm are improved using an oblique negative uniaxial film having aretardation of −25 nm.

Compared with FIG. 8, only the characteristics of the films 38 and 35have changed.

The compensation structure bonded to the face 31 comprises, for example,a first holographic film 35 having a retardation value of around −25 nm,angles θ and φ having values of 120° and 135° respectively, a Fuji film36 as described above having an angle φ equal to 315° and its CTA 37having a retardation value of −80 nm and an angle θ of 0.

The compensation structure bonded to the face 32 comprises, for example,a second holographic film 38 having a retardation value of around −25nm, angles θ and φ having values of 60° and 45° respectively, and a Fujifilm 39 as described above having an angle p with a value of 225° C. andits CTA 40 having a retardation value of −80 nm and an angle θ of 0.

FIG. 11 shows the isocontrast curves of the compensated cell describedin FIG. 10. Note there is an improvement over the conoscopes obtained inFIGS. 4 and 8.

FIG. 12 shows a variant in which the CTA of the Fuji film is improved,for example by modifying the value of its retardation.

FIG. 12 is an exploded view of a compensated liquid-crystal cell whichdiffers from the structure described in FIG. 10 by the CTA.

The CTA of the Fuji film in this example, labeled 37 and 40, has aretardation value of −110 nm.

The isocontrast curves obtained with such a structuring and given inFIG. 3 show an improvement over the conoscopes obtained above.

Various arrangements of the holographic film and of the film to beimproved may be imagined without departing from the scope of theinvention, some of which are given in the table below as an illustrationbut in no way implying any limitation.

Position of Fuji film Position of the hologram 1 The Fuji film is placedThe hologram is placed between a face of the between the polarizer andliquid-crystal cell and the Fuji film a face of a hologram 2 The Fujifilm is placed The hologram is placed between the polarizer between theliquid-crystal and the hologram cell and the Fuji film

Various arrangements of the compensation half-structures 1 and 2 aregiven in the table.

The liquid-crystal cell may be “flanked” on each side by a type-1 ortype-2 compensation structure.

It may also face the 1-type structure on one side and the 2-typestructure on the other side.

Without departing from the scope of the invention, the liquid-crystalcell may be of the twisted nematic (TN) type.

1. A display device, comprising; a liquid-crystal cell element placedbetween two polarizers, at least one of the polarizers including anoptical structure for compensating for variations in birefringence ofsaid liquid crystal cell element according to the viewing angle; whereinan optical compensation structure comprises an oblique-axis film ofpolymerized liquid-crystal for at least partly compensating forundesirable effects of natural birefringence of the liquid crystal,combined with a film compensatability characteristic improving volumehologram of small retardation configured for improving thecompensatability characteristics of said oblique axis film.
 2. Thedevice as claimed in claim 1, wherein said oblique-axis film is of anematic or polymerized discotic liquid-crystal type.
 3. The device asclaimed in claim 1, wherein said hologram is a holographic film havingan optical axis in the plane of the oblique-axis film.
 4. The device asclaimed in claim 1, wherein said hologram is a holographic film havingan optical axis tilted with respect to the plane of this film.
 5. Thedevice as claimed in claim 1, wherein a value of the retardation of saidvolume hologram is less than −150 nm, preferably between −10 and − 100nm.
 6. The device as claimed in claim 1, wherein a holographic assemblyincludes at least two holographic films each comprising a multilayergrating, the layers of which have their own orientation.
 7. The deviceas claimed in claim 1, wherein a first face of a liquid-crystal cell,which is configured to lie on the same side as an observer, is bonded toa first oblique-axis film which is itself bonded to a hologram and inthat a second face of the liquid-crystal cell is bonded to a secondoblique-axis film which is itself bonded to a hologram.
 8. The device asclaimed in claim 1, wherein a first face of the liquid-crystal cell,which is configured to lie on the same side as an observer, is bonded toa first hologram which is itself bonded to a first oblique-axis filmitself and in that a second face of the liquid-crystal cell is bonded toa second oblique-axis film which is itself bonded to a hologram.
 9. Thedevice as claimed in claim 1, wherein a first face of the liquid-crystalcell, which is configured to lie on the same side as an observer, isbonded to a first hologram which is itself bonded to a firstoblique-axis film itself and in that a second face of the liquid-crystalcell is bonded to a second hologram which is itself bonded to a secondoblique-axis film.
 10. The device as claimed in claim 1, wherein saidoblique-axis film comprises a stretched plastic film of the CTA typehaving a negative uniaxial birefringence and in that the retardationvalue of this film is tailored to the holographic film and to theoblique-axis film.
 11. The device as claimed in claim 10, wherein aliquid-crystal cell is of twisted nematic type.
 12. The device asclaimed in claim 11, wherein said device is used to compensate for thebirefringence effects in display devices, such as microcomputer screens.13. The device as claimed in claim 11, wherein a first face of aliquid-crystal cell, which is configured to lie on the same side as anobserver is bonded to a first oblique-axis film which is itself bondedto a hologram and in that a second face of the liquid-crystal cell isbonded to a second oblique-axis film which is itself bonded to ahologram.
 14. The device as claimed in claim 11, wherein a first face ofthe liquid-crystal cell, which is configured to lie on the same side asan observer, is bonded to a first hologram which is itself bonded to afirst oblique-axis film itself and in that a second face of theliquid-crystal cell is bonded to a second oblique-axis film which is iself bonded to a hologram.
 15. The device as claimed in claim 11,wherein a first face of the liquid-crystal cell, which is configured tolie on the same side as an observer is bonded to a first hologram whichis itself bonded to a first oblique-axis film itself and in that asecond face of the liquid-crystal cell is bonded to a second hologramwhich is itself bonded to a second oblique-axis film.
 16. The device asclaimed in claim 10, wherein a first face of a liquid-crystal cell,which is configured to lie on the same side as an observer, is bonded toa first oblique-axis film which is itself bonded to a hologram and inthat a second face of the liquid-crystal cell is bonded to a secondoblique-axis film which is itself bonded to a hologram.
 17. The deviceas claimed in claim 10, wherein a first face of the liquid-crystal cell,which is configured to the same side as an observer, is bonded to afirst hologram which is itself bonded to a first oblique-axis filmitself and in that a second face of the liquid-crystal cell is bonded toa second oblique-axis film which is itself bonded to a hologram.
 18. Thedevice as claimed in claim 10, wherein a first face of theliquid-crystal cell, which is configured to lie on the same side as anobserver, is bonded to a first hologram which is itself bonded to afirst oblique-axis film itself and in that a second face of theliquid-crystal cell is bonded to a second hologram which is itselfbonded to a second oblique-axis film.
 19. The device as claimed in claim1, wherein said oblique axis film comprises a multilayer of discoticmolecules and a negative uniaxial firm having n optical axisperpendicular to a retardation plane.
 20. A display device, comprising;polarizers disposed on either side of a liquid crystal cell element; anoptical structure associated with at feast one of the polarizers, theoptical structure being configured to compensate for variations inbirefringence of the liquid crystal cell element according to theviewing angle, and comprising: a polymerized liquid-crystal oblique-axisfilm configured to compensate for birefringence in the liquid crystalcell element, and a small retardation, film compensatabilitycharacteristic improving volume hologram recorded to have apredetermined relationship with respect to characteristics of theoblique-axis film and to modify the characteristics of the oblique-axisfilm.